Pros and Cons of Using DC/DC Converters to Reduce PA Battery Current

Whether you are sending e-mails with your smart phone, playing a game with a friend on your wireless tablet, or simply calling home, each wireless device represents a critical link in our connected world. With wireless connectivity being extended into mobile broadband by 3G and 4G technologies, the challenges that RF engineers face are rapidly evolving.

These challenges are overcome by device designers and engineers who are able to craft the RF section of each device to fit specific requirements and help create an optimal user experience. Before discussing battery life and power techniques for the power amplifier (PA), it is important to review the foundation of a working RF section. Therefore, the first step in any design is to ensure that the device is able to establish and maintain a high-quality wireless connection.

Key Signal Performance Requirements
The three main power amplifier criteria associated with connection quality are linearity, gain, and antenna power. Linearity is the ability of the power amplifier to accurately reproduce the frequency and amplitude variation in the RF input. This is an important specification, since it helps determine the ability of the device to maintain a robust wireless connection. The adjacent channel leakage ratio (ACLR) is often used as a measure of power amplifier linearity for modern wireless systems (Figure 1), such as WCDMA, HSPA and HSPA+. ACLR is defined as the ratio of the power in the adjacent channel to the power in the user channel.
Figure 1

Linearity specifies the behavior of the power amplifier to drive the output in a proportional manner to the input, and gain is the slope of this proportional curve. Fundamentally, gain represents the ratio of output to input power. The power amplifier must take the output of the transceiver, typically less than +3 dBm and amplify the signal without distortion.

Having selected a power amplifier that provides a linear signal with sufficient gain, RF designers must ensure that the end result is sufficient antenna power. The signal from the transceiver must pass through the power amplifier, switches, and filters, before reaching the antenna. Therefore, the power amplifier gain should provide sufficient RF power to overcome loss in other selected components and ensure antenna power of +24 dBm.
Current Consumption is Important
Achieving these key RF performance specifications is necessary, but it’s not sufficient in meeting the overall performance demands of the device. Users want to have a stable connection that allows for the maximum throughput available for their devices, however, they also want to ensure that battery life is able to meet their usage patterns. Therefore, with a properly designed RF section that is able to achieve optimal signal performance, the next challenge is meeting efficiency targets.

Mobile phones and other wireless devices operate at RF power levels that are determined both by the signal-to-noise ratio in their environment and the requirements of the network where they are attached. Typical operating power levels range from +24dBm while the phone is searching for a network connection to -20dBm or less while operating in regions of a cell with excellent SNR. A power distribution profile like the one published by the GSM Association in their DG09 procedure for measuring battery life is a good guide to the relative amounts of time that a WCDMA device spends at different power levels on a typical network [1]. The DG09 profile shows that mobile devices typically operate across a wide range of output power levels, with the greatest probability at low-range to mid-range power. While multi-function devices, such as smart phones, tend to operate at mid-range power levels, data devices tend to operate at higher output power levels.

Figure 2

With this power distribution defined, it is clear that optimizing the power amplifier’s current consumption at multiple power levels is important to prolonging battery life.

Power amplifiers used in early CDMA and WCDMA mobile phone designs had only one or two power modes. They were designed to be efficient at high output levels, but suffered from reduced efficiency at lower (backed-off) power levels.

Using a DC-DC converter or switched-mode power supply (SMPS) is a proven approach to reducing battery current in this kind of simple PA. The concept is to use a power supply that incorporates a switching regulator, in order to adjust the source for the power amplifier. The most common method of SMPS is the DC/DC converter, which converts the direct current (DC) voltage from the battery of the mobile device to a lower DC voltage level that is required by the power amplifier. The voltage supplied to the power amplifier is adjusted to the power requirements of the device in each environment. Therefore, if the power amplifier is in an environment where it does not need to operate at full output power, then a switched-mode DC/DC converter will reduce the voltage.